1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, to a structure for reducing noise when a sheet cassette detachably mounted on the apparatus main body is extracted.
2. Description of the Related Art
Nowadays, among image forming apparatuses, such as a copying machine, a printer, and a facsimile, widely used image forming apparatuses are configured in such a manner that a sheet feeding device feeds a sheet to an image forming portion to form an image. Generally in such an image forming apparatus, a sheet cassette is detachably mounted on the apparatus main body, and sheets stored in the sheet cassette are fed to the image forming portion by a feeding roller.
For example, there is a known sheet cassette in which a sheet stacking portion stacks sheets inside the cassette main body and the sheets are pressed to the feeding roller can be lifted and lowered. In addition, when a sheet is fed, the sheet stacking portion is lifted so that the sheet is pressed to the sheet feeding roller and thus the sheet is fed by virtue of a pressing force (hereinafter, referred to as a feeding pressure) between the feeding roller and the top surface of the sheet. In an exemplary image forming apparatus, a drive source for lifting/lowering the sheet stacking portion is provided in the apparatus main body side, and, when the sheet cassette is mounted on the apparatus main body, the drive source lifts the sheet stacking portion so that the sheet is pressed to the feeding roller, and a feeding pressure is generated.
However, some of the image forming apparatuses of the related art have a sheet feeding device which transfers the sheet from the drive source to a rotation shaft at a predetermined timing. Such sheet feeding devices include a drive gear and a tooth-chipped gear. The drive gear is connected to the drive source such as a motor, and the tooth-chipped gear meshes with the drive gear and is rotated by the changes of the clutch mechanism. In addition, when the sheet is fed, the tooth-chipped gear is rotated by the clutch mechanism so as to mesh with the drive gear. In this way, the feeding roller is rotated (refer to U.S. Pat. No. 6,070,867).
FIG. 18 illustrates such a sheet feeding device of the related art. The sheet feeding device includes a feeding roller 920 and an intermediate lifting/lowering cam 908 which are attached to the rotation shaft 905. The sheet feeding device further includes a liftable intermediate plate 900 which is upwardly biased by a biasing member (not illustrated) and is provided with protrusions 909 at both ends. In addition, as the rotation shaft 905 is rotated, the intermediate plate 900 is lifted by the protrusion 909 and the intermediate lifting/lowering cam 908 which rotates in synchronization with the rotation shaft 905, and the sheets S stacked on the intermediate plate 900 are pressed to the feeding roller 920. Then, as the feeding roller 920 is rotated, and in consequence the sheets S are fed.
However, the sheet feeding device includes a drive gear 901 and a tooth-chipped gear 902. The drive gear 901 is connected to the drive source such as a motor (not illustrated). The tooth-chipped gear 902 is fixed to the rotation shaft 905, has a clutching mechanism, and meshes with the drive gear 901 by the changes of the clutch mechanism. The solenoid 906 illustrated in FIG. 18 includes an armature 907, a coil 913 that generates magnetism when an electric current flows across it, a frame 910 that efficiently transmits the generated magnetism, and a stator 914 that generates a magnetic force as illustrated in FIGS. 19A and 19B. The armature 907 is attached to the support point 911 of the frame 910 and is biased in the direction of an arrow F by the biasing force of the spring 912.
The armature 907 has a locking portion 904 at the leading edge thereof, and the tooth-chipped gear 902 has a locking claw 915 which engages with the locking portion 904 of the armature 907. Here, the tooth-chipped gear 902 is biased to rotate in the direction of an arrow W by the biasing member (not illustrated). However, the tooth-chipped gear 902 is held at a position where the tooth-chipped gear 902 does not mesh with the drive gear 901 because the locking portion 904 of the armature 907 engages with the locking claw 915 as illustrated in FIG. 19A until sheet feeding is initiated.
Then, as an electric current flows to the solenoid 906 for the sheet feeding, a magnetic force is generated in the stator 914 due to the magnetism generated from the coil 913, and the armature 907 is attracted toward the stator 914 as illustrated in FIG. 19B. In this manner, as the armature 907 is attracted to the stator 914, the locking between the locking portion 904 of the armature 907 and the locking claw 915 of the tooth-chipped gear 902 is released, and the tooth-chipped gear 902 is rotated in the direction of the arrow W and meshes with the drive gear 901.
As a result, the tooth-chipped gear 902 is rotated by the drive gear 901, which brings about the rotation of the rotation shaft 905 engaged with the tooth-chipped gear 902, and in consequence the intermediate lifting/lowering cam 908 integrated with the rotation shaft 905 becomes rotated. In this manner, as the intermediate lifting/lowering cam 908 is rotated, the intermediate plate 900 is lifted by the protrusions 909 provided at both ends of the intermediate plate 900, and the sheets S stacked on the intermediate plate 900 are is pressed to the feeding roller 920. In addition, since the tooth-chipped gear 902 and the feeding roller 920 are fixed to the rotation shaft together, the feeding roller 920 is rotated along with the tooth-chipped gear 902, thereby feeding the sheets S that are pressed to the feeding roller 920.
However, in such an image forming apparatus of the related art, the armature 907 of the solenoid 906 is locked when the apparatus is not used, and the position of the tooth-chipped gear 902 is held. Therefore, the tooth-chipped gear 902 does not mesh with the drive gear 901. However, if a strong vibration or impact is applied, for example, when the apparatus is transported after the apparatus is manufactured, or when the image forming apparatus is moved, the armature 907 of the solenoid 906 may be released from the locking claw 915 of the tooth-chipped gear 902.
If the armature 907 of the solenoid 906 is released as described above, the tooth-chipped gear 902 meshes with the drive gear 901. In this case, the sheet may be fed just by rotating the motor even when the solenoid is not driven during use of the apparatus, and therefore, it may generate a sheet jam and consume sheets uselessly.